965 



GEOLOGY. 



GEOLOGY. 



966 



deposition of sedimentary matter, and the slow development of organu 

 life. If many writers, and Cuvier himself in the number, still con 

 tinned to maintain that ' the thread of induction was broken,' yet in 

 reasoning by the strict rules of induction from recent to fossil species 

 they in a great measure disclaimed the dogma which in theory they 

 professed. The adoption of the same generic, and in some cases even 

 of the same specific, names for the exuviae of fossil animals and their 

 living analogues, was an important step towards familiarising the 

 mind with the idea of the identity and unity of the system in distau 

 eras. It was an acknowledgment, as it were, that part at least of the 

 ancient memorials of nature were written in a living language. The 

 growing importance then of the natural history of organic remains 

 may be pointed out as the characteristic feature of the progress o 

 the science during the present century. This branch of knowledge has 

 already become an instrument of great utility in geological classifica 

 tion, and is continuing daily to unfold new data for grand and enlarged 

 views respecting the former changes of the earth. When we compare 

 the result of observations in the last fifty years with those of the three 

 preceding centuries, we cannot but look forward with the most 

 sanguine expectations to the degree of excellence to which geology 

 may be earned, even bj the labours of the present generation. Never 

 perhaps did any science, with the except^fn of astronomy, unfold in 

 an equally brief period so many novel and unexpected truths, and 

 overturn so many preconceived opinions. The senses had for ages 

 declared the world to be at rest, until the astronomer taught that it 

 was carried through space with inconceivable rapidity. In like manner 

 was the surface of this planet regarded as having remained unaltered 

 since its creation, until the geologist proved that it had been the 

 theatre of reiterated change, and was still the subject of slow but 

 never-ending fluctuations. The discovery of other systems in the 

 boundless regions of space was the triumph of astronomy to trace 

 the same system through various transformations to behold it at 

 successive eras adorned with different hills and valleys, lakes and seas, 

 and peopled with new inhabitants, was the delightful meed of geolo- 

 gical research. By the geometer were measured the regions of space 

 and the relative distances of the heavenly bodies by the geologist 

 myriads of ages were reckoned, not by arithmetical computation, but 

 by a train of physical events signs which convey to our minds more 

 definite ideas than figures can do of the immensity of time." 



Geology is distinct from cosmogony. The history of the successive 

 phenomena happening on a planet revolving round an orb of light 

 and heat may be treated without reference to the condition of the 

 same material particles while they were subject to entirely different 

 conditions. Yet as in tracing the progress of a colony reference may 

 often be made with advantage to the previous history of the same 

 people in another region of the globe, so, in prosecuting geological 

 science in a just and liberal sense, it is advisable to take into account 

 the discoveries of collateral science, so far as these tend to give sure 

 indications of, or even to fix certain limits to, speculations concerning 

 the origin of the planetary masses. 



For tbe successful prosecution of this inquiry geology must appeal 

 to two entirely distinct branches of collateral science, chemistry, and 

 astronomy ; which indeed agree in this, that they are both directed 

 to the elucidation of the properties of material substance ; but the 

 former is occupied with a study of its elementary constitution, the 

 latter contemplates the relations of its congregated masses. 



Chemisty, by analysis of the different sorts of matter visible near 

 the surface of the earth, teaches us that almost everything is of a 

 compound nature, and formed by the union of two or more elementary 

 particles, endowed with distinguishable properties, and capable of a 

 separate existence and of entering into new combinations. When 

 thus freed from their combinations by processes of art the elementary 

 particles or atoms, of the same kind, form, when reunited, solids, 

 liquids, or gaseous expansions, according as they are affected by tempera- 

 ture, pressure, and perhaps other less general influences. Oxygen, 

 the most abundant of all the elementary substances yet discovered, 

 expands immediately on being freed from union with solid bodies, to 

 a gas which occupies 2000 times the space it previously did ; and as 

 nearly half the ponderable matter of the globe consists of oxygen, 

 we must admit, as a plain consequence of this analysis, that upon a 

 general resolution of the compound rocks and minerals into their 

 constituent elements, nearly half the weight of the exterior parts of 

 the globe would expand into gas, and augment the atmosphere till the 

 accumulated pressure should liquify the gas, or prevent further decom- 

 position. What happens to free oxygen with the temperatures and 

 atmospheric pressures which now prevail at the surface, would (we 

 know by trial) happen to chlorine and other substances similarly 

 released from combination, under other temperatures and pressures. 

 As these conditions are now variable, and may be supposed to have 

 passed through all possible grades, it is not improbable that all the 

 substances which exist in the crust of the globe might be converted 

 into gaseous expansions if freed from combination. The great 

 antagonist force to the concentration of matter is heat ; by augment- 

 ing this agent some substances are decomposed and the parts rendered 

 volatile ; in other cases combinations take place which are also 

 volatile ; aud there are others in which gaseous substances combine 

 with solids at particular temperatures only. Now, as the substances 

 known in the outer parts of the globe are about 60 in number, as they 



all separately stand in different relations to heat, pressure, electricity, 

 &c., it is conceivable that under particular conditions the mutual 

 forces of the various particles might be so arranged, and so balanced 

 by the influences of heat and other general conditions, that all sensible 

 solidity and liquidity should vanish, -and the whole globe dissolve 

 into an expansion, where the particles would be, if not all free, yet in 

 very different combinations from those we now see. This is con- ' 

 ceivable as an hypothesis, and chemistry can teach us no more ; for 

 as we have not ascertained for each substance, taken singly, what 

 must be the conditions for its appearance as a solid, liquid, or gaseous 

 body, nor have the means of computing what variation in this respect 

 might result from particular admixtures of the substances, it is 

 impossible to deny that the hypothesis may be true, and it would be 

 equally uuphilosophical to assert that it is. In this dilemma we must 

 turn to the contemplation of phenomena which may serve to guide 

 us to a just decision. Omitting for the present all considerations of 

 geological phenomena, we must accompany the astronomer in his 

 survey of space, in order to discover if any masses of matter exist 

 which are of the nature of the gaseous expansion assumed ; if this 

 be the case, we must further inquire if there be gradations in the 

 appearances they present such as to justify the belief in the possibility 

 of a gradual conversion of a planet into an expansion, or the contrary. 

 To these inquiries the far-seeing eyes of Herschel supply a positive 

 answer. Through various parts of the heavens are scattered large 

 expansions of attenuated matter, called nebulae, which are irregularly 

 reflective of light, various in figure and degree of condensation. The 

 latter circumstances being carefully studied, it appears that many of 

 them are of a globular or elliptical figure, as if the parts were 

 collected by a general attraction toward a centre; that others in 

 addition, appear to grow continually denser toward a centre, while 

 not a few objects show in the centre the brightness of a solid star 

 surrounded by a thick and extensive haze. Occasionally two or more 

 points of condensation appear in a nebulous mass, thus affording a 

 great analogy with what may be supposed to be the origin of our 

 planetary system. 



Comets, which are to be regarded as nebula; attracted to some one 

 or more systems, supply another and strong analogy with orbitual 

 planets. But it may be reasonably expected that in addition to the 

 graduated appearances of expansion, condensation, and nebulous 

 solidity, there should be proof of corresponding gradations of density. 

 This proof, as far as relates to the nebulae far distant from our system, 

 can perhaps never be given, though appearances are in favour of the 

 view; even with respect to the comets which enter the solar system, 

 further researches must be made ; but the planets themselves supply 

 such a proof, for their density varies exceedingly. The planets 

 nearer to the sun are denser than those farther removed ; Mercury, 

 being the heaviest, is almost thrice as dense as the earth, while 

 Jupiter, one of the distant orbs, is about one-third as dense as our 

 earth ; aud Saturn, which, excepting Uranus, is the most remote, is 

 only one-eighth or one-tenth as dense, and may be considered as light 

 as cork. (Herschel, ' Introduc. to Astron.,' p. 278.) 



Finally, this general idea of the origin of the mass of the earth 

 from a nebular expansion, suggested by chemical facts, and supported 

 by the appearances in the visible heavens, is confirmed by the mathe- 

 matical researches of Laplace, who has by this supposition connected 

 together the most striking phenomena of the solar system ; the 

 jeueral parallelism of the orbits of the planets, the consentaneous 

 direction of their movement round the sun, of the satellites round 

 ihe planets, the anomaly of Saturn's ring, and other important circum- 

 stances. We have therefore only one test more to which the hypo- 

 thesis can be subjected, namely, its accordance with what is known 

 of the actual constitution of the earth. This is still no question of 

 ijeology, but of astronomy. It appears however very, certain that 

 neither the figure of the earth, which is that of a spheroid of revo- 

 ution on its axis, nor the density of the earth, which is greater 

 ;oward the centre than at the circumference, and so arranged that 

 the surfaces of equal density are symmetrical to the axis of figure, 

 are at all opposed to the doctrine in question, but rather confirm it. 

 From astronomical and chemical considerations, then, it is probable 

 ihat the mass of the earth once existed as ti part of a diffused nebula, 

 ike some now visible in the heavens ; and as no merely geological 

 evidence as to the changes operated on the condensed planet can be 

 of the smallest value in a question relating to the coudeusatiou of a 

 nebula, we must adopt the conclusion as a limiting condition of 

 ;eological theory. 



But however firmly we may admit the truth of the speculation of 

 ;he condensation of planets from a nebular expansion, it can now 

 lave but little influence on the progress of geology. For it cannot 

 >e employed as the origin of deductions which mignt disclose circum- 

 tances hidden from observation in deep parts of the earth, and 

 explain complicated facts visible at the surface ; and this for want of 

 adequate knowledge of the successive effects which must happen 

 among the elementary particles or masses of a nebula during its 

 soudeusation, as well as of the uecessaVy consequences whicii such 

 3fTects must entail on the physical conditions of a planet. 



There is however one point of importance which this speculation, 

 f adopted, may assure us of. The condensation of nebulte is gradual ; 

 he density of planets various the larger ones in general having the 



